As a braking device for land transportation vehicles such as railway vehicles, automobiles, and motorcycles, disc brakes have been widely used in recent years as those vehicles have become increasingly faster and larger. A disc brake is a device that produces a braking force by means of friction generated by sliding contact between a brake disc and a brake lining.
In the case of railway vehicles, a braking force is generated by pressing a brake lining, by a brake caliper, against a frictional surface of a brake disc in the form of a donut-shaped disc which has been mounted and fixed to the wheel or the axle. In this manner, the rotation of the wheel or the axle is slowed or stopped so that the speed of the vehicle is controlled.
During the operation, deceleration of the vehicle depends on the braking force of the disc brake, and the braking force greatly depends on the coefficient of friction between the brake disc and the brake lining. In actual traveling, it is desired that deceleration of a vehicle under braking can be accurately controlled, and in order to achieve this, the coefficient of friction between the brake disc and the brake lining should desirably be stable and constant regardless of variations in the traveling speed at the time of start of the braking operation.
In addition, during the braking operation, the temperatures of the frictional surfaces of the brake lining and the brake disc that come into contact with each other increase due to frictional heat. The temperature increase tends to be noticeable under conditions where the braking load increases, specifically when the vehicle is travelling at high speed or the vehicle is heavy. In actual traveling, it is desired that thermal damage to the disc brake be prevented so that its durability is enhanced. In order to achieve this, it is necessary to provide as uniform contact as possible between the brake lining and the brake disc during the braking operation and reduce the generation of frictional heat.
FIG. 1 shows a conventional typical disc brake for railway vehicles. In the figure, FIG. 1(a) is a planar view of a brake lining, and FIG. 1(b) is an enlarged cross sectional view of FIG. 1(a) taken along line A-A. FIG. 1(a) shows the brake lining as seen from the brake disc side, which is the front side.
The conventional typical brake lining (hereinafter referred to as a “conventional brake lining”) 102 shown in FIG. 1 includes: a plurality of wide plate-shaped friction members 103; metallic backings 104 secured to the back surfaces of the friction members 103; and a base plate 106 holding the friction members 103 together with the metallic backings 104 on the back surface side. The friction members 103, together with the metallic backings 104, are rigidly mounted to the base plate 106 by means of rivets (not shown). In the thus configured conventional brake lining 102, the base plate 106 is mounted to a brake caliper (not shown), so that the front surfaces of the friction members 103 face the frictional surface 101a of the brake disc 101.
The brake caliper is actuated by, for example, hydraulic or pneumatic pressure as a driving source during the braking operation, to press the brake lining 102 against the brake disc 101. During this operation, the pressing force applied from the brake caliper to the brake lining 102 does not act uniformly on the entire area of the brake lining 102, but acts on some particular regions in a concentrated manner due to the mounting structure of the two parts.
As a result of this, in the case of a disc brake including the conventional brake lining 102, the contact pressure between the brake disc 101 and the brake lining 102 locally increases during the braking operation because the friction members 103 have a fairly wide plate shape and are rigidly fixed to the base plate 106. Consequently, the temperature increase due to the friction during the braking operation becomes locally excessive, which results in an increase in wear of the brake lining 102 (friction members 103) and the brake disc 101 or the formation of cracks in the brake disc 101 at the location experiencing the excessive temperature increase. This thermal damage leads to the loss of durability of the disc brake.
To address this problem, a variety of brake linings with an improved structure have been proposed in recent years with the intention of providing uniform contact pressure between the brake lining and the brake disc during braking operation. For example, Patent Literatures 1 to 3 describe brake linings in each of which friction members are provided in such a manner that the friction member of the conventional brake lining is divided into smaller pieces and the plurality of divided pieces of friction members are arranged to be spaced from each other.
In the brake lining described in Patent Literature 1, the friction members are each separately provided with a metallic backing secured to the back surface thereof. The friction members together with the metallic backings are mounted, on the back surface side, via separately provided spring members, to a base plate by means of rivets, so that they are separately resiliently supported.
In the brake lining described in Patent Literature 2, the friction members are provided in pairs with each pair being formed by two adjacent ones of the friction members, and the paired friction members are together provided with a one-piece resilient leaf secured to the back surfaces thereof. Each resilient leaf is rigidly mounted to the base plate by means of two rivets at a portion corresponding to a gap between the friction members fixed to the resilient leaf. In this manner, the friction members are each separately resiliently supported at the two locations of the rivets between the friction members serving as supporting points.
In the brake lining described in Patent Literature 3, the friction members are provided in pairs with each pair being formed by two adjacent ones of the friction members, and the paired friction members are together provided with a one-piece metallic thin film secured to the back surfaces thereof. Each metallic thin film is attached to the base plate by means of two rivets via elastic elements at a portion corresponding to a gap between the friction members that are secured to the metallic thin film. In this manner, the friction members are each separately resiliently supported at the two locations of the rivets between the friction members serving as supporting points.
All the brake linings described in Patent Literatures 1 to 3 are expected to have uniform contact pressure with respect to the brake disc during the braking operation because their friction members are resiliently supported on the base plate.
However, the brake lining described in Patent Literature 1 is configured such that the friction members are each independently movable. Because of this, the coefficient of friction between the brake disc and the brake lining may vary depending on the traveling speed at the time of start of the braking operation. Such variability of the friction coefficient occurs in the brake linings described in Patent Literatures 2 and 3 as well. This is because the friction members in those literatures too are each independently movable because they are each resiliently supported via the resilient leaf or the metallic thin film at the location between the paired friction members serving as a supporting point.
If the coefficient of friction varies depending on the traveling speed at the time of start of the braking operation, accurate control of deceleration of the vehicle under braking becomes difficult, thus making it impossible to ensure the required braking performance. In particular, in the case of high speed railway vehicles such as a Shinkansen “bullet train”, the variation in the coefficient of friction poses a problem because the traveling speed broadly ranges from low speed to high speed.
Furthermore, in the case of the brake lining described in Patent Literature 1, the friction members are each fastened to the base plate by a single rivet, and thus they are caused to rotate at their respective positions during the braking operation. If such rotation of the friction members is repeated as a result of repeated braking operations, loosening occurs at the fastening portion between the friction members and the base plate, which finally results in falling off of the friction members. Because of this, it is impossible to ensure sufficient durability and reliability.